This project is in continuation of Project GM 07212-13. It would be a terminal grant, since the principal investigator will retire from the University of Melbourne on 12/31/1975 (he will however, continue this work at the Australian National University after that date). It is proposed to study the nature and adaptive significance of the cytogenetic events which have occurred during the differentiation of populations, races and species. These include inversions, translocations and other rearrangements of major chromosome segments, but they also involve heterochromatinization of smaller sections. Our main material is the all-female parthenogenetic grasshopper Moraba virgo a diploid species which is a complex heterozygote for major chromosomal rearrangements and also for a number of late replicating DNA segments. Hybridizing this species with males of related bisexual species yields triploid hybrids of both sexes which are used for autoradiography, thereby providing comparisons between the replication patterns of the parental species within the same cell. Studies of enzyme polymorphism in different populations of M. virgo and especially the biometrical studies of W.R. Atchley will provide information on genetic divergence of the populations which it is hoped to compare with minor differences in DNA replication pattern. The fact that the triploid hybrids include y-bearing males suggests, but does not prove, that the neo-Y chromosomes of the species 'P169' and 'P196' are male-determining. Large-scale attempts to obtain hybrids between virgo and other related species whose male are XO will be carried out to clarify the mechanisms of sex determination. In addition it is proposed to study the nature and cytogenetic significance of the unique accessory chromosome which exist in some species of the Australian roach genus Polyzosteria, in addition to the autosomes and X-chromosomes. These accessory chromosomes differ from supernumerary or B-chromosomes in being invariably present and are hence presumably essential for life.